Level Tracking in Thermal-Hydraulic Simulations of Nuclear Reactors

Open Access
Author:
Aktas, Birol
Graduate Program:
Nuclear Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 17, 2003
Committee Members:
  • John Harlan Mahaffy, Committee Chair
  • Kostadin Nikolov Ivanov, Committee Member
  • Gordon Edward Robinson, Committee Member
  • Christopher J Duffy, Committee Member
Keywords:
  • level tracking
  • computational fluid mechanics
  • two-phase flows
  • thermal-hydraulics
  • reactor safety
Abstract:
The presence of stratified liquid-gas interfaces in vertical flows poses difficulties to most classes of solution methods for two-phase flows of practical interest in the field of reactor safety and thermal-hydraulics. These difficulties can plague the reactor simulations unless handled with proper care. To illustrate these difficulties, the US NRC Consolidated Thermal-hydraulics Code (TRAC-M) was exercised with selected numerical benchmark problems. These numerical benchmarks demonstrate that the use of an average void fraction for computational volumes simulating vertical flows is inadequate when these volumes consist of stratified liquid-gas interfaces. An accurate description of these computational volumes, which are divided into two regions with distinct flow topology, requires that separate void fractions be assigned to each region. This strategy requires that the liquid-gas interfaces be tracked in order to determine their location, the volumes of regions separated by the interface, and the void fractions in these regions. Although the idea of tracking stratified liquid-gas interfaces is not new, its applications in the reactor safety codes in the past showed only limited success. Improper modifications to the field equations were mostly responsible for their failures. This thesis proposes a systematic approach to implement a method for tracking interfaces in the one- and three-dimensional field equations of TRAC-M. This approach is applicable to the solution methods of known thermal-hydraulic codes of the same class. The success of this approach was demonstrated by exercising TRAC-M with the same benchmark problems that were previously used to expose the difficulties of handling the liquid-gas interfaces.